The present invention is directed on wear protecting workpieces, as especially hard metal or steel workpieces, thereby especially of tools, especially cutting tools by means of wear-resistant and thus wear-protective layer systems.
Definitions:
A layer system as used in the present description and claims shall mean any number of layers deposited one upon the other and may be realized by a single layer too.
We understand under xe2x80x9chard metalxe2x80x9d cemented carbides.
When we refer to xe2x80x9cpredominantly consisting of a materialxe2x80x9d this shall mean by at least 50 at %, especially by at least 90 at %.
The present invention relates to a workpiece which comprises a workpiece body and a wear-resistant layer system on at least a part of the surface of said workpiece body, said wear-resistant workpiece having an excellent wear resistance as concerns abrasion and oxidation resistance and adhesion of the layer system. Thereby, the workpiece is in a most preferred form a tool, and especially a cutting tool, as e.g. an end mill.
Wear-resistant layer system comprising at least one layer provided on the body surface of TiN or TiCN or the like have hitherto been widely and generally used especially for tools. However, investigations have recently been made on incorporation of Al to the hard material layer for improvements in wear resistance, thereby of abrasion resistance and of oxidation resistance. Thus, there are various cases where the effects of the addition of Al to the above mentioned hard materials has S been exploited, as shown in the Japanese Patent Publication No. 53,642/1992 and the Japanese Patent Publication No. 67,705/1993.
In these cases, however, only improvements of wear resistance of the film itself, in terms of oxidation resistance and of abrasion resistance, has been attained by addition of Al. Nevertheless, wear resistance incurs more, namely the resistance of the wear resistant layer against being peeled off the base body. i.e. adhesion.
With a recent strong tendency to increase the efficiency of cutting, the cutting speed and the feed rates for cutting tend to be increased. In such cases, the adhesion of the hard material layer, i.e. its resistance against being peeled or chipped off, becomes extremely important as a more and more dominant factor in the life span of a tool, which factor even dominates the mere abrasion resistance and oxidation resistance of the layer itself. The above mentioned known layers of hard material and containing Al are generally so high in residual compression stress that the adhesion of the layer is not satisfactory, resulting in too frequent peeling off during high efficiency cutting. This spoils the life span and reliability of the tool considerably.
Obviously, we speak of the problems encountered with cutting tools, but equal problems will arise generally at tools and even more generally at workpieces, if exposed to similar loading as cutting tools. Thus, if we refer to problems and solutions referring to cutting tools, this is valid also for other tools and more generically other workpieces similarly exposed to mechanical, thermal and chemical stress.
In order to realise stable cutting with a long life span of a tool, even during high efficiency cutting, the adhesion of the wear resistance layer system must be further enhanced.
Investigations have been made on decreasing the residual compression stressxe2x80x94as a root cause of deterioration in adhesionxe2x80x94in the hard material of the hard material layer with the target of improving thereby the adhesion, As a matter of fact, however, satisfactory results are yet to be obtained.
It is an object of the present invention to provide a wear-resistant workpiece which comprises
a workpiece body
a wear-resistant layer system on at least a part of said body and comprising at least one hard material layer, which is provided closest to said body
said hard material layer at least predominantly consisting of at least one material selected from the group consisting of;
Nitride, carbonitride, carbonitrooxide, nitroboride and carbonitroboride of at least titanium and aluminum,
which wear-resistant workpiece has an improved wear resistance with respect to abrasion resistance, oxidation resistance and,
and especially, to adhesion of the wear-resistant layer system, i.e. has an improved adhesion of the hard material layer.
This object is resolved by inventively providing an intermediate layer between the body and the hard material layer which intermediate layer predominantly consists of at least one material selected from the group
Nitroboride, carbonitroboride, carbonitrooxide of at least one of chromium, tantalum, zirconium, titanium,
Nitride, carbonitride of at least one of chromium, tantalum, zirconium.
As a result of intensive investigation with a view of improving the adhesion of the hard material layer to the workpiece body, the inventors of the present invention have found out that the comparatively soft intermediate layer when provided under the Al-containing hard material layer, which latter has a high compression stress, absorbs and hence relaxes the high compression stress of the hard material layer. Thereby, the adhesion of the hard material layer is remarkably improved. This suggests that the high compression stress when present in the hard material layer develops a high shear stress workingxe2x80x94without intermediate layerxe2x80x94in the interface between the hard material layer and the workpiece body. This shear stress is an important factor in spoiling the adhesion of the hard material layer, The inventively realized relaxation by inventively providing the intermediate layer removes this shear stress at the interface and thereby improves the adhesion of the hard material layer, In other words, it is believed that the inventively provided intermediate layer as a comparatively soft layer, interposed between the hard material layer and the body, absorbs and hence relaxes the shear stress formerly developed in the interface when the hard material layer was directly deposited on the workpiece body. Therefrom it becomes clear that the intermediate layer is to be provided generically xe2x80x9cbetweenxe2x80x9d the body and the hard material layer, irrespective how many and which material layers are additionally provided between the body and the intermediate layer on one hand and the intermediate layer and the hard material layer on the other hand. Nevertheless, it is preferred, with one exception of a metal layer as discussed below, to interpose the intermediate layer directly to the hard material layer and the body surface.
The object of still further improving wear resistance of the inventive wear-resistant workpiece is resolved by the present invention in that the intermediate layer and/or the hard material layer consists of at least one material selected from nitroborides, carbonitroborides.
As for the conditions required for the intermediate layer it has been recognised that not only softness, but also comparatively coarse crystal grains are favorable for stress relaxation. A better surface roughness of the intermediate layer favorably results in an improvement of adhesion. Although elucidation of other factors leading to further improvements require further studies, it has been found out that the best results are realized with the above mentioned intermediate layer materials.
It has also been confirmed, that addition of boron to the hard material and/or intermediate layer, and especially to the latter, leads to a further remarkable improvement in respect of surface roughness and provides even more favorable results with respect to adhesion of the hard material layer, Elucidation of this phenomenon may also require further studies.
Whereas conventional examples of intermediate layers make use of intermediate layer material predominantly consisting of TiN, TiCN or TiC, the present studies made by the inventors of the present invention however have shown that these well-known materials first of all involve formation of a layer having a rough surface and fine crystal grains, but which is not very effective in absorbing and relaxing shear stress as being not sufficiently soft. They result in a notably lower effect of improving adhesion as compared with the intermediate layer of the inventively proposed materials.
In a preferred form, the workpiece comprises a hard material layer of TiAlN and an intermediate layer of nitroboride or of carbonitroboride of titanium, further preferably directly interposed between body and hard material layer or realized with an additional metal layer as outlined below.
It is namely a further object of the present invention to still additionally improve wear resistance of the workpiece being provided with the inventive intermediate layer.
This is accomplished in that the workpiece further comprises a metal layer which is interposed between the base body and the intermediate layer. Thereby, in a most preferred form of realisation, the metal layer predominantly consists of at least one of chromium, tantalum, titanium and zirconium and, further preferred, predominantly consists of the same metal which is provided as a component in the inventively provided intermediate layer.
The inventors of the present invention have thereby recognised that the shear stress of the inventive workpiece can be further relaxed by providing a soft metal layer under the intermediate layer. This is based on the fact that a metal layer is high in absorption energy, low in Young""s Modulus and easy of shift of dislocation and is thereby additionally effective in absorbing strain energy. Thereby, in a preferred form the workpiece comprises a hard material layer of TiAlN and an intermediate layer of nitroboride or of carbonitroboride of titanium, and further a metal layer interposed between the body and the intermediate layer which at least predominantly consists of at least one of chromium, tantalum, zirconium or titanium, further preferably of chromium and/or titanium, preferred of chromium or titanium. It is thereby preferred to directly interpose the metal layer to the body and the intermediate layer, latter being provided just adjacent the hard material layer.
As a result, peeling off of the hard material layer is inventively suppressed, even in intermittent heavy cutting at a feed rate exceeding 0.4 mm per blade, whereby stable cutting can be realized. As was mentioned above, the inventive workpiece in all its embodiments has preferably a workpiece body of a hard metal or of a steel as of a high-speed steel. It is thereby preferably realized as a tool and especially as a cutting tool.
For the thickness of the intermediate layer, d, there preferably prevails:
0.05 xcexcmxe2x89xa6dxe2x89xa65 xcexcm.
This is based on the following reasons:
When the thickness of the inventively provided intermediate layer is not at least 0.05 xcexcm, it may be ineffective in relaxing the stress, i.e. in improving the adhesion of the hard material layer. On the other hand, when the thickness d of the intermediate layer exceeds 5 xcexcm the abrasion resistance of the hard material layer may be spoiled.
With respect to the metal layer which isxe2x80x94in a preferred form of realisationxe2x80x94provided at the inventive workpiece, the thickness dm of such metal layer is preferably selected to be
5 nmxe2x89xa6dmxe2x89xa6500 nm.
This in based on the following reasons:
If on one hand the thickness of the metal layer interposed between the intermediate layer and the base body is not at least 5 nm, it may likewise be ineffective in further relaxing the stress, i.e. in improving the adhesion of the hard material layer. On the other hand, when it exceeds 500 nm, plastic deformation may be developed in this metal layer during cutting, resulting in spoiling the adhesion of the hard material layer.
It is a further object of the present invention to provide a method for producing a wear-resistant workpiece, which results in such workpiece with significantly improved wear resistance. This method comprises the steps of depositing above the surface of the body of the workpiece an intermediate layer predominantly consisting of at least one material selected from the group of
Nitroboride, carbonitroboride, carbonitrooxide of at least one of chromium, tantalum, titanium, zirconium,
Nitride, carbonitride of at least one of chromium, tantalum, zirconium
and then depositing above said intermediate layer a wear-resistant layer system by depositing above said intermediate layer a hard material layer predominantly of at least one material selected from the group of
Nitride, carbonitride, carbonitrooxide, nitroboride and carbonitroboride of titanium and aluminum,
thereby depositing the intermediate layer as well as the hard material layer by means of a vacuum coating process.